1. Field of the Invention
The present invention relates generally to design systems and methods, and more particularly to an LCD (Liquid Crystal Display) pixel circuit design system and method.
2. Description of Related Art
A color filter liquid crystal display (CF-LCD) uses a white backlight module such as a cold cathode fluorescent lamp (CCFL) to provide a broad-spectrum light source with various wavelengths, which is selectively filtered in terms of color and position when passing through pixels of the display. In general, a pixel is composed of three sub-pixels, and the electric field intensity of each sub-pixel is controlled by a field-effect transistor (FET) so as to determine the light intensity passing through the sub-pixel. Using this design, the broad-spectrum light source passing through the sub-pixels is selectively filtered into primary color light corresponding to the sub-pixels by primary color filters corresponding to the sub-pixels respectively. The lights of primary colors are then mixed by the human eyes into a specific color associated with the pixel.
A field-sequential color liquid crystal display (FSC-LCD) directly modifies the light source configuration of the backlight module by replacing the white backlight module used in the CF-LCD with three primary color light sources. Therefore, the FSC-LCD no longer needs the color filters, and there is no need to divide each pixel into sub-pixels. In operation, the formation of color of the FSC-LCD is performed by modulating the light emitting clocks of three primary color light sources in the backlight module. Meanwhile, the FETs of the pixels are coordinated by the corresponding clocks to control the electric field intensity of the pixels so as to determine the light intensity passing through the pixels. As a result, the relative intensity of the primary color lights is regulated by synchronously controlling LCD pixel transmission. After the three primary color lights enter the viewer's eyes, the visual system performs an integration operation on the received light stimulus to mix the primary color lights into a single color matching the predetermined color of the pixel.
The field-sequential imaging technique and related products have already existed. For example, digital light processing (DLP) projectors have adopted this technique. However, some bottlenecks must be overcome before this technique is applicable to an LCD display.
In the display technology industry, frames need to be displayed at a rate above 60 Hz to meet the lowest changing rate required by the human visual system to successfully display a complete image, particularly a moving image. Because an CF-LCD uses a white backlight module and three primary color sub-pixel light filters for modulation, the CF-LCD can provide three primary color light sources at the same time and accordingly operate at the lowest changing rate (60 Hz). However, because an FSC-LCD uses three primary color time-changing backlight modules to replace a broad-spectrum light source, the switching frequency for the three primary color sources has to be three times that of the CF systems using broad-spectrum light source. In other words, the lowest changing rate of the FSC-LCD system must be 60 Hz multiplied by 3, i.e., 180 Hz so as to meet the lowest changing rate required by the human visual system to successfully display a complete image.
If the synchronization or response frequency of FSC-LCD is poor, color breakup (CBU) effect will appear and adversely affect the visual impression. From the above, it can be seen that finding a way to increase the changing rate is one of the most challenging issues involved in applying the field-sequential color imaging technique to LCD displays.
To increase the changing rate of an LCD system, not only the changing rate of the backlight but also the response of the display pixel circuit must meet the requirement of the lowest changing rate. In 1996, Tsukada proposed a design theory for LCD pixel circuits in “TFT/LCD Liquid-Crystal Displays Addressed by Thin-Film Transistors,” 2nd ed., Taylor & Francis, 2000, which takes into consideration the charging behavior, capacitor coupling effect and signal delay in designing the pixel circuits. Further, Y. H. Tai proposed in 2006 the concept of an operation window based on charging, holding, coupling and delay in “Design and Operation of TFT-LCD Panels,” WuNan, 2006.
In addition to meeting the requirement of the lowest changing rate, taking into consideration the light transmission ratio of a pixel is another important design concept in designing an LCD pixel circuit. In the operation of an LCD, the amount of light passing through a pixel is controlled by voltage. And, if the pixel has a large light transmission area, the pixel can attain high light transmission so as to achieve a preferred display effect and reach a specific color and luminance standard with low power consumption and low cost. Y. Kaneko, A. Sasano, and T. Tsukada proposed in 1989 an equivalent circuit model used in designing pixel circuits of an LCD (“Analysis and design of a-Si TFT/LCD panels with a pixel model,” IEEE Trans. Electron Dev., vol. 36, no. 12, pp. 2953-2958, December 1989). According to the equivalent circuit model, persons skilled in the art not only can ascertain the basic structure of a pixel circuit, but also can see that the pixel circuit has a large portion of light-impervious area.
In designing products, design conditions and constraints need to be considered at the same time. For example, comprehensively considering the design conditions and constraints such as charging/discharging behavior, voltage potential holding, capacitor coupling effect and signal delay is one of the most important challenges in designing a pixel circuit. However, until now, no design system has ever considered the above design parameters of the display pixel circuit at the same time to calculate and compare performance variations of a variety of designs with different design parameters, thereby helping a designer to select ideal design parameters in designing a display pixel circuit.
Therefore, there is a need to provide a display design system and method that can calculate and compare performance variations of a variety of designs corresponding to different design parameters so as to help a designer to select ideal design parameters for designing a display pixel circuit.